IQ is probably a measure of intelligence. Lately, scholars have proffered the notion of intellect as a complex phenomenon, the call this the many intelligences theory. I don't think this theory negates the fact that IQ measures something which is, or is essential to intelligent activity. Whether intelligence is due primarily to nature or nurture is another matter altogether.
Intelligence quotient tests are excellent at measuring how well people perform on intelligence quotient tests. That is the limit of what an IQ test can really measure. These results can be extrapolated to show with some reliability how successful people may be at certain activities, but they are not enough to truly measure a person's intelligence (if such a thing can even be measured). An IQ test would have told you nothing about Picasso's ability with a paint brush, or Mozart's innate musical skills, and to say that you do not need intelligence to do what Picasso and Mozart did seems to stretch credulity.
I believe intelligence can be measure by IQ tests, but it can't give the whole picture. I believe intelligence can come in many different forms and there are also people who perform poorly on tests. As with any test, it should be used as a guide, not a definitive answer.
The idea that there is one test out there that can properly quantify into a number value is ridiculous. If some one said there was a battery of tests you take ranging from the standard IQ test to physical problem solving I would be more prone to agree with that measure of mental equity.